Method for manufacturing SOI wafer and thus-manufactured SOI wafer

ABSTRACT

A silicon oxide film  3′, 3 ″ is formed on each of the main surfaces of a first silicon single crystal substrate  1  (bond wafer) and a second silicon single crystal substrate  2  (base wafer), and the first and second silicon single crystal substrates are then brought into close contact so as to locate the silicon oxide films  3′, 3 ″ in between in an atmosphere of a clean air supplied through a boron-releasable filter, to thereby produce an SOI wafer  10 . The second silicon single crystal substrate  2  employed herein comprises a silicon single crystal substrate having a bulk resistivity of 100 Ω·cm or above. In thus produced SOI wafer  10 , the silicon oxide film  3  has a depth profile of boron concentration in which the boron concentration reaches maximum at a thickness-wise position. This ensures manufacturing of SOI wafer excellent in high-frequency characteristics.

FIELD OF THE INVENTION

[0001] The present invention relates to a method for manufacturing SOIwafer and thus—manufactured SOI wafer.

BACKGROUND OF THE INVENTION

[0002] There has been a general trend of handling high-frequency signalof several hundred MHz or above in recent mobile communication typicallyusing cellular telephones, which strongly demands semiconductor deviceswith excellent high-frequency characteristics. Semiconductor devicessuch as CMOS-IC and high-voltage IC typically employ so-called SOI wafercomprising a silicon single crystal substrate (also referred to as “basewafer” hereinafter), a silicon oxide layer (buried oxide film) formedthereon, and another silicon single crystal layer stacked furtherthereon as an SOI (silicon-on-insulator) layer. For the purpose offabricating semiconductor devices for high-frequency use on the SOIwafer, it is necessary for the base wafer to be composed of ahigh-resistivity silicon single crystal in order to reduce highfrequency loss.

[0003] One representative process for manufacturing the SOI waferrelates to bonding process. According to the bonding process, a firstsilicon single crystal substrate (also referred to as “bond wafer”hereinafter), which provides an SOI layer affording device formationarea, and a second silicon single crystal substrate which serves as abase wafer are bonded so as to locate a silicon oxide film in between,and the bond wafer is then reduced in the thickness thereof so as to bethinned to a film having a predetermined thickness, to thereby convertthe bond wafer to the SOI layer.

[0004] In the above-described bonding process, a bonding interfacebetween the base wafer and the bond wafer may sometimes catch foreignmatters such as particles. Such foreign matters accidentally residing onthe bonding interface may induce lattice defect such as void, degradedwafer characteristics typically due to diffusion of impurities, anddegraded bonding strength between both substrates. The substrates arethus bonded in a clean room (or in a clean area) so as to avoid thecontamination of foreign matters into the bonding interface. In themanufacture of SOI wafer by the bonding process, it is a generalpractice to form the silicon oxide film only on the surface of the bondwafer, and then bond the base wafer with the bond wafer so as to locatethe silicon oxide film in between.

[0005] Another known problem resides in that the clean room, which is asite of the wafer bonding, usually contains in the atmosphere thereofboron which is derived from the air filter, and which boron can beincorporated as an impurity into the bonding interface. Boron thusincorporated into the bonding interface diffuses during high-temperatureannealing (bonding annealing) for raising bonding strength or duringannealing for forming devices. In this point of view, the foregoingbonding process in which the silicon oxide film is formed only on thebond wafer hardly affects the devices since the boron diffusion into theSOI layer (device forming area) is blocked by the silicon oxide film.This is one reason why the foregoing bonding process in which the bondwafer, only on which the silicon oxide film is formed, is bonded withthe base wafer is widely accepted. Whereas the bonding interface betweenthe base wafer and silicon oxide film still suffers from adsorption ofboron derived from the air filter, so that the boron diffusion into thebase wafer is still inevitable during the foregoing bond-annealing.

[0006] The above-described boron diffusion into the base wafer has notattracted much attention so far as a silicon single crystal substratehaving a normal-to-low resistivity is used as the base wafer. Theproblem of degradation of high-frequency characteristics however arisesin the SOI wafer for high-frequency use, since the base wafer has aresistivity of as high as hundreds to thousands Ω·cm, and theresistivity of an interfacial portion of the base wafer severalmicrometers deep from the interface with the silicon oxide film mayconsiderably be lowered due to the boron diffusion.

[0007] One solution for the foregoing problem is disclosed in UnexaminedJapanese Patent Publication No. 2000-100676, in which SOI wafer ismanufactured by properly selecting types of the air filter used forintroducing air into a clean room to thereby control the amounts ofboron as a p-type impurity together with n-type impurity in the bondingatmosphere. The methods disclosed in the patent are such that:

[0008] 1. using a boron-free filter system which comprises a PTFE filterand a boron-adsorptive chemical filter irrespective of conductivity typeof the base wafer. Using the boron-free filter is beneficial to suppressboron-induced degradation in resistivity of the base wafer particularlyfor the case that the base wafer comprises a p-type silicon singlecrystal substrate having a high resistivity; and

[0009] 2. using a boron-releasable HEPA filter when the base wafercomprises an n-type silicon single crystal substrate having a highresistivity. Degradation of the resistivity is avoidable even if boronis adsorbed since the adsorbed boron is compensated by the n-type dopantcontained in the n-type silicon single crystal substrate.

[0010] The foregoing method 1 is however disadvantageous in that theboron-free filter system is expensive and is less economical. While themethod 2 is applicable to the case the n-type base wafer is used, it isof course inapplicable to the case the p-type base wafer is used. Theparagraph 0150 of the foregoing patent publication also describesdifficulty in use of the HEPA filter for the high-resistivity, p-typewafer. It is also anticipated that even the resistivity of the n-typewafer may degrade unless concentrations of the n-type dopant and thefilter-derived adsorbed boron are properly balanced.

SUMMARY OF THE INVENTION

[0011] An object of the present invention therefore resides in providinga method for manufacturing SOI wafer less causative of degradation ofresistivity of the base wafer even when a high-resistivity siliconsingle crystal substrate of either conductivity type is used as the basewafer and is bonded in a boron-containing atmosphere; and also inproviding an SOI wafer producible by such method, capable of retaininghigh resistivity of the base wafer by localizing boron incorporatedduring the bonding, capable of retaining desirable electricalcharacteristics of the SOI layer, and excellent in high-frequencycharacteristics.

[0012] To solve the foregoing problem, the method for manufacturing SOIwafer of the present invention comprises a bonding step including aprocess of bringing the main surfaces of a first silicon single crystalsubstrate and a second silicon single crystal substrate, each of suchmain surfaces having previously formed thereon a silicon oxide film,into close contact so as to locate such silicon oxide films in between;and a thickness reducing step for reducing the thickness of such firstsilicon single crystal substrate to thereby convert it into an SOIlayer, wherein such second silicon single crystal substrate comprises asilicon single crystal substrate having a bulk resistivity of 100 Ω·cmor above, and such process of bringing the main surfaces into closecontact in such bonding step is proceeded in an atmosphere of a cleanair supplied through a boron-releasable filter.

[0013] The present invention employs a silicon single crystal substratehaving a bulk resistivity of 100 Ω·cm or above as the second siliconsingle crystal substrate (corresponded to the base wafer), and dareemploys, in order to bring such substrate into close contact, anatmosphere containing a high concentration of boron derived from the airfilter, which is usually found in ordinary clean rooms. The atmosphereis composed of a clean air supplied through a boron-releasable filter(which is exemplified by HEPA filters disclosed in Unexamined JapanesePatent Publications Nos. 10-165730 and 8-24551). In the presentinvention, the silicon oxide film is respectively formed on both of thesecond silicon single crystal substrate and the first silicon singlecrystal substrate (corresponded to the bond wafer), and both siliconoxide films are then brought into contact with each other.

[0014] The SOI wafer of the present invention comprises a silicon singlecrystal substrate; a silicon oxide film formed on the main surface ofsuch silicon single crystal substrate; and an SOI layer comprising asilicon single crystal layer formed on such silicon oxide film, whereinsuch silicon single crystal substrate has a bulk resistivity of 100 Ω·cmor above, and such silicon oxide film has a depth profile of boronconcentration in which the boron concentration reaches maximum at athickness-wise position more closer to the silicon single crystalsubstrate away from the center of the film thickness.

[0015] According to the method for manufacturing SOI wafer of thepresent invention, the bonding interface is formed within the siliconoxide film, which means that boron which resides in the bondingatmosphere is confined within the silicon oxide film (buried oxidefilm). Since the diffusion coefficient of boron in the silicon oxidefilm is small, the boron diffusion into the SOI layer and silicon singlecrystal substrate (base wafer) can successfully be suppressed even afterhigh-temperature annealing for raising bonding strength of the oxidefilms.

[0016] It is preferable herein that the thickness of the silicon oxidefilm formed on the base wafer is smaller than the oxide film formed onthe bond wafer. By manufacturing the bonded SOI wafer based on suchdefinition of the thickness of the oxide films on both wafers, thebonding interface is formed at a thickness-wise position more closer tothe base wafer away from the center of the film thickness. This ensuresthe SOI wafer to have more stable device characteristics. The nextparagraphs will describe the reason why.

[0017] To prevent the high-frequency characteristics of the SOI waferfrom being degraded, it is necessary to avoid lowering of resistivity ofthe base wafer as described in the above. The present invention thusprovides an effective measure whereby the oxide films are mutuallybonded so as to confine the atmospheric boron into such oxide films.There is, however, still an apprehension that boron confined in thebonding interface may diffuse in the oxide film to reach the SOI layeror base wafer depending on various conditions such as boronconcentration in the bonding atmosphere, bonding annealing, annealingfor device fabrication, and thickness of buried oxide film necessary fordevice characteristics.

[0018] The concentration of boron possibly diffused through the oxidefilm might be fairly small as compared with the boron concentration inthe vicinity of the bonding interface as described in the above, buteven such small amount of boron can adversely affect the devicecharacteristics ensured by the SOI layer if diffuses thereto, since theabsolute amount of dopant intrinsically contained in the SOI layer asthin as 1 μm or less is quite small. Moreover, when there is a need forthe thickness of buried oxide layer of as thin as 0.1 μm or less,thinner buried oxide film makes the bonding interface closer to the SOIlayer. In a microscopic view, the bonding interface, however, has sitesof incomplete chemical bond, and fixed charge ascribable to such sitesmay adversely affect the SOI layer in which device formation area willbe reserved. Considering the above, the bonding interface is preferablyformed in the buried oxide film closer to the base wafer.

[0019] On the other hand, the base wafer will suffer from only a slightdegree of lowering in the bulk resistivity thereof and will cause only afairly limited degradation of the high-frequency characteristics if aslight amount of boron diffused through the oxide film may beincorporated therein, since the absolute value of the dopantconcentration of such base wafer is relatively large, despite its highresistivity, if a large thickness thereof is taken into consideration.It is also noteworthy that the fixed charge within the buried oxide filmwill never affect the base wafer which does not serve as active layer ofdevices.

[0020] As judged from the above, the thickness of the oxide film formedon the bond wafer is preferably 0.1 μm or more in consideration ofeffects on the SOI layer exerted by boron diffused through the oxidefilm, or by fixed charge which resides in the bonding interface. Thethickness of the oxide film exceeding 2 μm is, however, not practicalsince formation of such thick film needs a considerably long annealingtime in a normal-pressure thermal oxidation furnace which is widelyaccepted.

[0021] The SOI wafer of the present invention will therefore be suchthat effectively suppressing the boron-induced lowering of the bulkresistivity of the silicon single crystal substrate, and beingpreferably applicable to high-frequency devices. It is also economicalsince use of an expensive facility such as boron-free filter system isno more necessary.

[0022] As the second silicon single crystal substrate (referred to as asilicon single crystal substrate in the bonded SOI wafer) employed inthe method of the present invention, it is preferable to use a substratehaving a resistivity of 100 Ω·cm or above, more preferably 500 Ω·cm orabove, and still more preferably 1,000 Ω·cm or above in view of ensuringdesirable high-frequency characteristics.

[0023] The bonding step in the method of the present invention mayinclude an annealing process which is carried out within a temperaturerange from 1, 150 to 1, 300° C. so as to achieve sufficient bondingstrength. For the purpose of bonding the oxide films with each other,annealing at a temperature below 1,150° C. may sometimes result ininsufficient bonding strength. More specifically, the SOI wafer obtainedafter annealing below 1,150° C. may show only an insufficient chemicalbonding strength when measured by immersing such wafer into an aqueoussolution of hydrofluoric acid so as to assess the corrosion status, evenafter a sufficient mechanical bonding strength is observed typically intensile strength test. On the other hand, annealing at 1,150° C. orabove, preferably at 1,200° C. or above, ensures a satisfactory level ofbonding strength not only mechanically but also chemically. Theannealing temperature exceeding 1,300° C., however, will be more likelyto generate slip dislocation, which is inappropriate since problems indurability of an annealing furnace and metal contamination tend toarise. The annealing temperature is thus preferably set at 1,250° C. orbelow from a practical viewpoint.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1A is a drawing for explaining process of manufacturing theSOI wafer of the present invention;

[0025]FIG. 1B is a drawing as continued from FIG. 1A;

[0026]FIG. 1C is a drawing as continued from FIG. 1B;

[0027]FIG. 1D is a drawing as continued from FIG. 1C;

[0028]FIG. 2 is a schematic drawing of a clean room whereat the bondingstep in the method for manufacturing SOI wafer according to the presentinvention is carried out;

[0029]FIG. 3 is an explanatory chart showing the SOI wafer of thepresent invention and a depth profile of the boron concentration in thesilicon oxide film;

[0030]FIG. 4 is a graph showing a relation between exposure time ofwafer and concentration of boron deposited on the surface of the wafer(surface density);

[0031]FIG. 5 is a drawing for explaining a mechanism according to whichboron is incorporated into the silicon oxide film in the method of thepresent invention; and

[0032]FIG. 6 is a graph showing measured boron concentration in anExample and Comparative Example.

BEST MODE FOR CARRYING OUT THE INVENTION

[0033] Preferred embodiments of the present invention will be describedhereinafter.

[0034]FIGS. 1A to 1D are drawings for schematically explaining themethod for manufacturing SOI wafer according to the present invention.First as shown in FIG. 1A, silicon oxide films 3′, 3″ are formed on mainsurfaces 1 a, 2 a of a bond wafer 1 as a first silicon single crystalsubstrate and a base wafer 2 as a second silicon single crystalsubstrate, respectively. The silicon oxide films can be formed not onlyby wet oxidation but also by dry process such as CVD (chemical vapordeposition) or the like. It is preferable that the silicon oxide film 3′on the bond wafer 1 is adjusted so as to have a thickness of 0.1 to 2μm, and the silicon oxide film 3″ on the base wafer 2 is formed so as tobe thinner than the silicon oxide film 3′ on the bond wafer 1. As thebase wafer 2 herein a silicon single crystal substrate with a highresistivity (specifically 100 Ω·cm or above) is used. While the bondwafer 1 is not specifically limited, a substrate having a normal rangeof resistivity (approx. 1 to 20 Ω·cm) is generally used.

[0035] Next, at least the surfaces having formed thereon the siliconoxide films 3′, 3″ of the bond wafer 1 and base wafer 2 are cleanedusing a cleaning solution, and the both wafers are then, as shown inFIG. 1B, brought into close contact on their sides where the siliconoxide films 3′, 3″ are formed at room temperature or around, andannealed in an annealing furnace at 1,150 to 1,300° C. to therebytightly bond them with each other. Such bonding step can be carried outin a clean bench 21 housed in a clean room 20 as shown in FIG. 2. Theinner atmosphere of the clean room 20 and clean bench 21 typicallycomprises a clean air supplied through a boron-releasable filter 22 suchas HEPA filter. The saturated concentration value of boron deposited onthe surface of the wafer, which is left in the clean bench 21 or theclean room 20, generally falls within a range from 10¹² to 10¹³atoms/cm².

[0036] By the annealing, the silicon oxide films 3′, 3″ are united toform a silicon oxide film 3 as shown in FIG. 1C, and by being interposedwith such silicon oxide film 3 the bond wafer 1 and base wafer 2 aretightly bonded. The bonding interface 4 is thus formed within thesilicon oxide film 3. Next, the bond wafer 1 is thinned to a targetedthinning plane 5 shown in FIG. 1C so as to leave the bond wafer 1 in athickness sufficient for forming devices. After the thinning process, anSOI layer 6 having a predetermined thickness is remained as shown inFIG. 1D. The thinning process for reducing the thickness of the bondwafer 1 can be effected by various methods, and is by no meansspecifically limited in the present invention. One exemplary methodrelates to reduction in the thickness of the bond wafer 1 by grindingand polishing it from a plane opposite to the plane having alreadyformed thereon the silicon oxide film 3 (referred to as “polishingprocess” hereinafter). For the purpose of further thinning after thepolishing, a dry etching technique called PACE (plasma-assisted chemicaletching) can typically be applied. It is a general strategy to combinethese methods to thin the wafer to the targeted thinning plane 5.

[0037] Methods other than the foregoing polishing process includeso-called Smart-Cut process (registered trademark). In this process, thebond wafer 1 is implanted with ions of a light-weight element such ashydrogen, helium, or the like prior to the bonding, then brought intoclose contact with the base wafer 2 so as to locate the oxide film inbetween, which is followed by annealing. The bond wafer 1 is separatedat the portion where the light-weight element ions are implanted, tothereby give the SOI layer 6 of a predetermined thickness which servesas a device forming area. The Smart-Cut process is advantageous in thata separated portion of the bond wafer 1 obtained after the annealing canbe recycled as a new bond wafer or base wafer. It is to be noted,however, that this process is advantageous in obtaining a relativelythin SOI layer, and it is difficult to produce a relatively thick SOIlayer (typically 1 μm thick or above) since the depth of implantation ofthe light-weight element ions is as small as 0.1 to 1 μm or around.

[0038] Recent development efforts also resulted in a method in which theions to be implanted after being excited in plasma, to thereby allow theseparation at room temperature or around without any special annealing.Annealing for the separation will thus be omissible when such method isadopted to the present invention.

[0039] According to the foregoing processes, an SOI wafer 10 of thepresent invention as shown in FIG. 1D is obtained. In thus produced SOIwafer 10, the silicon oxide film 3 will have formed therein anintermediate position along the thickness-wise direction (athickness-wise position closer to the base wafer away from the center ofthe thickness in this embodiment) whereat the boron concentrationreaches maximum as shown in FIG. 3. This is because, as shown in FIG. 5,boron B previously adheres on the surface of the silicon oxide film 3prior to the bonding, and the bonding is carried out while retaining theadhesion status, which allows boron B to be incorporated within thesilicon oxide film. Boron concentration will be maximum in the vicinityof the bonding interface 4. The position whereat the boron concentrationreaches maximum can vary depending on the relation between thickness ofthe silicon oxide film 3′ formed on the bond wafer 1 and the siliconoxide film 3″ formed on the base wafer 2. When the thickness of thesilicon oxide film 3′ on the bond wafer 1 and the thickness of thesilicon oxide film 3″ on the base wafer 2 are almost equivalent, thebonding interface 4 is formed approximately at the center of the unitedsilicon oxide film 3, so that the position whereat the boronconcentration reaches maximum also falls approximately on the center.However, the position whereat the boron concentration reaches maximum ispreferably closer to the base wafer 2 based on the foregoing reason.

EXAMPLE AND COMPARATIVE EXAMPLE

[0040] The following experiment was carried out in order to confirm theeffects of the present invention. Silicon single crystal substrateswhich serve as a bond wafer and a base wafer were sliced out fromsilicon single crystal ingots pulled by the MCZ (magnetic-field-appliedCzochralski) method. The bond wafer employed herein was a p-type siliconsingle crystal substrate having a diameter of 200 mm, a resistivity of10 Ω·cm, an interstitial oxygen concentration of 12 ppma (based on thestandards by JEIDA (Japanese Electronic Industry DevelopmentAssociation)), a thickness of 725 μm and a crystal orientation of <100>,and the base wafer employed herein was a p-type silicon single crystalsubstrate having a diameter of 200 mm, a resistivity of 1,200 Ω·cm, aninterstitial oxygen concentration of 6 ppma (JEIDA standards), athickness of 725 μm and a crystal orientation of <100>.

[0041] On the individual main surfaces of the foregoing base wafer andbond wafer, the silicon oxide films were formed by the method describedbelow. On the bond wafer, a silicon oxide film of 0.5 μm thick wasformed by wet oxidation under annealing conditions of 1,050° C. for 120minutes, and on the base wafer, a silicon oxide film of 0.1 μm thick wasformed by wet oxidation under annealing conditions of 800° C. for 100minutes.

[0042] The bond wafer having thus formed thereon the silicon oxide filmwas then implanted with hydrogen ions, where an ion acceleration energywas 46 keV, and an amount of dose was 8×10¹⁶/cm². The bond wafer andbase wafer were then subjected to SC-1 cleaning, and stacked with eachother in a clean bench of a clean room having an atmosphere purified bya HEPA filter at room temperature, to thereby bring them into closecontact. A relation between exposure time of wafer and concentration ofboron deposited on the surface of the wafer (surface density) was asshown in FIG. 4, from which the exposure time in this experiment wasdetermined as 60 minutes.

[0043] Thus stacked wafers were annealed at 500° C. for 30 minutes tothereby bond them, and concomitantly the bond wafer was allowed tocleave at the hydrogen ion implanted layer to thereby produce the SOIlayer of approx. 0.3±0.005 μm thick. The substrate having formed thereonthe SOI layer was then annealed at 1,200° C. for 60 minutes for thepurpose of enhancing the bonding strength of the bonding interface tothereby obtain an SOI wafer (Example). On the other hand in ComparativeExample, another SOI wafer was manufactured similarly to Example exceptthat the silicon oxide film was formed only on the bond wafer in athickness of 0.6 μm.

[0044] The SOI layers of thus produced SOI wafers in Example andComparative Example were removed by alkali etching, and depth profile ofthe boron concentration of each buried oxide film from the surfacethereof along the sectional-thickness-wise direction was then measuredby SIMS (secondary ion mass spectroscopy). It was confirmed from themeasurement that, in Example, the boron concentration reached maximum atapprox. 0.5 μm deep from the surface of the buried oxide film, whichposition corresponds to the bonding interface or around. On thecontrary, the boron concentration of buried oxide film in ComparativeExample only showed a slight increase in the vicinity of bondinginterface, and the concentration in other area was below the lowerdetection limit. The depth profile of boron concentration was measuredby SIMS also for the base wafer from the surface thereof after theburied oxide film was etched off using an aqueous HF solution. Resultswere shown in FIG. 6. While the SOI wafer of Comparative Example showedincrease in the boron concentration (that is, reduction in theresistivity) towards the surficial portion of the base wafer, it wasconfirmed that the SOI wafer of Example showed the boron concentrationin the surficial portion almost kept constant.

1. A method for manufacturing SOI wafer comprising: a bonding stepincluding a process of bringing the main surfaces of a first siliconsingle crystal substrate and a second silicon single crystal substrate,each of the main surfaces having previously formed thereon a siliconoxide film, into close contact so as to locate the silicon oxide filmsin between; and a thickness reducing step for reducing the thickness ofthe first silicon single crystal substrate to thereby convert it into anSOI layer, wherein the second silicon single crystal substrate comprisesa silicon single crystal substrate having a bulk resistivity of 100 Ω·cmor above, and the process of bringing the main surfaces into closecontact in the bonding step is proceeded in an atmosphere of a clean airsupplied through a boron-releasable filter.
 2. The method formanufacturing SOI wafer according to claim 1, wherein the thickness ofthe silicon oxide film formed on the main surface of the second siliconsingle crystal substrate is smaller than that of the silicon oxide filmformed on the main surface of the first silicon single crystalsubstrate.
 3. The method for manufacturing SOI wafer according to claim1, wherein the thickness of the silicon oxide film formed on the mainsurface of the first silicon single crystal substrate is 0.1 to 2 μm. 4.The method for manufacturing SOI wafer according to claim 1, wherein thebonding step includes an annealing process carried out within atemperature range from 1,150 to 1,250° C.
 5. (cancel)
 6. The method formanufacturing SOI wafer according to claim 2, wherein the thickness ofthe silicon oxide film formed on the main surface of the first siliconsingle crystal substrate is 0.1 to 2 μm.
 7. The method for manufacturingSOI wafer according to claim 2, wherein the bonding step includes anannealing process carried out within a temperature range from 1,150 to1,250° C.
 8. The method for manufacturing SOI wafer according to claim3, wherein the bonding step includes an annealing process carried outwithin a temperature range from 1,150 to 1,250° C.
 9. The method formanufacturing SOI wafer according to claim 6, wherein the bonding stepincludes an annealing process carried out within a temperature rangefrom 1,150 to 1,250° C.